Any book entitled Agatha Christie's Secret Notebooks promises a treasure trove of mystery gems. The shiniest jewels in this collection are a pair of long-hidden short stories featuring Hercule Poirot—the detective hero of 33 Christie novels (and 51 stories) published from 1920 to 1975, including some of her iconic works: The Mysterious Affair at Styles, The Murder of Roger Ackroyd, Murder on the Orient Express, and Death on the Nile.

Of the two stories debuting here—"The Incident of the Dog's Ball" and "The Capture of Cerberus"—the latter is by far the richer discovery. "Cerberus" reveals new aspects of the Christie canon and offers a glimpse at the era's publishing values and concerns. In 1947 Christie published her best-known short-story collection, The Labours of Hercules—12 mysteries paralleling the adventures of the Greek hero whose name is echoed in Poirot's. Eleven of these originally appeared in The Strand, an English mystery magazine, from 1939 to 1940. The 12th story, "Cerberus," did not.

That omission had been a mystery of its own—until now.

In the 1947 version of "Cerberus," Poirot descended into a London nightclub named Hell to expose a drug ring. Not so in the original draft, which had lain hidden in plain sight for more than six decades—just another bound typescript in the Christie archives, one of a number that had been "lifted and carried and moved and re-shelved numerous times over that period" until Curran, a literary advisor to the Christie estate, happened to sit down and read the pages during preparations for restoring Dame Agatha's house.

Instead of seamy-side London, that earlier draft takes readers into the heart of Continental Europe to focus on the assassination of August Hertzlein, a thinly disguised Adolf Hitler. While on vacation in Switzerland, Poirot is approached by the father of the man who allegedly assassinated Hertzlein—and who in turn had been killed by an angry mob. Convinced that his son could not have committed such a crime, the father asks Poirot to uncover the truth. Poirot agrees, then dispatches a succession of men to what he enigmatically labels "the place of departed spirits"—and soon a telegram arrives back with an address and the phrase "Beware of the Dog." When Poirot himself sets out to investigate, he discovers that—

Ah, but that would spoil the surprise! Suffice it to say that the story reveals the deeply political concerns of an author who elsewhere insisted that she had "never been in the least interested in politics." It also underscores the reluctance of British publishers to print an edgy political tale in a genre traditionally considered escapist literature.

These two "lost" stories appear only at the very end of Agatha Christie's Secret Notebooks. I read them (as many readers will) before leafing back to the notebooks themselves. Christie's mysteries, after all, promise more enjoyment than do studies of her style. But Curran's scholarship entailed detective work too, for he has followed clues spread across the 73 notebooks Christie left at her death to re-create her working habits and plumb the secrets of her success.

One might imagine these notebooks as regal, hardbound journals, but as Curran points out they "look like the piles of exercise books gathered by teachers at the end of class in schools the world over." He found them stuffed in a cardboard box in an upstairs room of the Christie family home. And they are about as far from systematically organized as one writer could get: a single notebook often spans decades. Plans for one novel crop up randomly across several notebooks. And the overall mix is leavened (some would say obscured) by random to-do lists, travel directions, and bridge scores.